Speed of the nerve impulse Aim: To recap work done last lesson!

Nerve impulses – key facts A nerve impulse is the movement of an action potential Nerve impulses are always a fixed size and speed in a given neurone Speed 0.5ms-1 – 120ms-1

Factors affecting speed of action potential Presence of myelin sheath Diameter: Temperature:

Presence of myelin sheath Impulse faster with in a myelinated axon than a unmyelinated axon What term is used to describe conduction in a myelinated axon? saltatory conduction!

Diameter Larger diameter, faster impulse Why? Larger diameter relatively less leakage of ions therefore membrane potentials easier to maintain. Narrower = greater resistance In thinner axons the localised circuits form in a much smaller area therefore conduction is slower.

Temperature The higher the temperature the faster the impulse … Up to a point! Why? Higher temperature gives a faster rate of diffusion Energy for active transport from respiration, which is dependant on enzymes therefore higher temp faster rate of reaction. Too high a temp … enzymes and plasma membrane proteins denature … No more impulses!

Refractory period After impulse sodium voltage gated channels remain closed for a period of time. Therefore its impossible to generate another action potential

Purpose of refractory period Only propagated in one direction. The action potential cannot be propagated in a region that is refractory so it can only move forwards Ensures action potentials separated. This limits the number of action potentials in a given time

Extra info Actually there are 2 refractory periods! Absolute refractory period – Lasts 1ms and no impulses can pass Relative refractory period – Lasts 5ms. New impulse only if the stimulus is more intense than the normal threshold

All or nothing principle Impulse only triggered at a certain level of stimulus, this is called the threshold value The impulse is always the same size and speed

How do organisms determine the size of the stimulus? Larger stimulus: More impulses in a given time Different Neurones with different threshold values